Foreign troops must stay in the strife-torn Central African Republic to stop it from turning into another Mali-style crisis, a top UN envoy said Thursday.

Margaret Vogt, the United Nations special envoy in the country where rebels control about half the territory, said troops — provided by neighbouring nations, South Africa and France — had become a "firewall" against the spread of insecurity.

With rebels reported to have taken new towns in spite of a ceasefire, Ms Vogt warned "this place can easily become another Mali," where French forces have intervened to halt an Islamist advance on the capital.

The foreign troops must help "to create a firewall in the CAR to prevent the trends we see from the Sahel from coming through here," Vogt told reporters.

Countries from the Economic Community of Central African States (ECCAS) hurriedly sent troops to the Central African Republic this month as rebels bore down on the capital Bangui having taken large parts of the country.

A ceasefire accord between President Francois Bozize and the Seleka coalition rebels was signed in Libreville on January 11. While a national unity government has been formed, rebel forces have failed to start a promised withdrawal from the streets of the towns they control.

Ms Vogt told reporters via video link from Bangui that security remains tense, with UN premises ransacked in four towns and other international organizations also looted.

UN agencies have said the rebels are using child soldiers and have entered two new eastern towns, Dimbi and Kembe, since the ceasefire.

The Seleka rebels, opponents of Mr Bozize who accuse him of failing to carry out an earlier peace accord, remain 75 kilometres (45 miles) from the capital.

Ms Vogt said "the international community now needs to engage more forcefully, both diplomatically and financially, to pull the CAR from the brink."

"The position of the CAR is strategic," Ms Vogt said.

"Whatever happens in the CAR will have an impact on all the countries in the region," added the UN envoy. The African governments who sent troops know "the situation can also undermine the security of their own countries."